It is known in the oil and gas industry to use a top drive motor and a drill-string to drill wells. It is the top drive motor that provides the torque to rotate the drill-string, which in turn rotates the drill bit at the bottom of the well. The drill-string itself consists of a series of hollow pipes, typically 30 ft (9.14 m) in length, and these are attached to each other via a threaded connection. The top drive is also attached to the drill-string via a threaded connection.
During the drilling process, drilling-mud is pumped through the connection between the top drive and the drill-string. This drilling-mud travels through the drill-string and ensures sufficient lubrication, cooling and the removal of cuttings. It is often necessary to remove the drill-string from the well (to replace the drill bit for example) and under such circumstances drilling-mud is pumped through the drill-string to displace and support the retreating drill-string and maintain hydraulic balance in the well bore. This ensures that a vacuum is not created and that the force required to remove the drill-string is minimised, allowing the removal to occur more quickly. In a conventional arrangement, the drilling-mud is pumped through the same connection, between the top drive and drill-string, as used when drilling.
When removing a drill-string from a well (which in the industry is known as tripping-out), successive sections of the drill-string have to be disconnected from the remaining sections of the drill-string. Furthermore, the section being removed also has to be disconnected from the top drive. A new connection is then established between the top drive and the remaining sections of the drill-string. However, making and breaking these threaded connections is-very time consuming and slows down the process of removing a drill-string from a well. This has a serious impact on the productivity of the well.
Previous attempts have been made at speeding up the process of tripping-out. GB2156402A discloses methods for controlling the rate of withdrawal and the drilling-mud pressure to maximise the tripping-out speed. However, the time taken to connect and disconnect each section of the drill-string to the top drive is not addressed. Other attempts include removing several sections at a time, as discussed in GB2156402A. However, this approach is limited by the height of the derrick holding the top drive.